Ca-bearing Rectorite Mineral from Roseki Deposits, Japan

Several specimens of rectorite-type regularly interstratified mineral (including Ca-rich one of hydrother-mal origin) from roseki deposits together with a few rectorites were examined by chemical analysis, X-ray diffraction and IR spectroscopy. These specimens are rich in SiO2, AI2O3 and H2O in addition to considerable amounts of CaO, Na2O and K2O. The Ca content in mica-like layers ranges from a slight amount to exceeding 50% of the total interlayer population. For the Ca-bearing specimens, AI-for-Si substitution in the tetrahedral sheets of mica-like layers increases in proportion to the Ca contents. Characteristic absorption bands of 950-900 cm-1 and 700-670 cm-1 are observed in IR spectra. The bands are due to AI-for-Si substitution in the tetrahedral sheets. Judging from the chemical analysis and IR spectra, the component of mica-like layer rich in Ca is not that of mica but mainly that od margarite (brittle mica). Expansion characteristics of the Ca-rich mineral are similar to rectorite and expandable layer is close to beidellite. The mineral is somewhat less expandable than rectorite under the condition of RH70-80% (Na-saturation) and EG treatment (K-saturation). The rectorite-type Ca-rich mineral reported here is simlar to rectorite in its expansion characteristics, but the component of mica-like layers is different from that of rectorite. Mica-like layer of the mineral must be mainly composed of margarite-like layer. The mineral is mainly composed of a 1 : 1 regular interstratification of dioctahedral britlle mica (margarite) and smectite (beidellite). The mineral does not strictly correspond to rectorite, defined as a 1 : 1 interstratified mineral of dioctahedral mica and smectite.